The present invention relates to a semiconductor laser and a method of fabricating the same. More particularly, the present invention relates to a semiconductor laser using an InGaAlBN based material and a method of fabricating the same.
Recently, development of a semiconductor laser using an InGaAlBN based material has been driven as a short wavelength light source, which is required for an optical disc with a higher recording density or the like. A semiconductor laser made of this kind of material can emit a beam having a small diameter in its adaptation to a short wavelength and therefore it is hoped that the laser is put into practice as a light source for high-density information processing such as an optical disc. A semiconductor laser using a multi-quantum-well-structure, as a structure realizing oscillation by current injection in this material system, has been reported, for example, in the following articles:
1) S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku and Y. Sugimoto: xe2x80x9cInGaN-based multi-quantum-well-structure laser diodesxe2x80x9d, Jpn. J. Appl. Phys., 35 (1996) pp. L74-L76.
2) S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku and Y. Sugimoto: xe2x80x9cInGaN multi-quantum-well-structure laser diodes with cleaved mirror facetsxe2x80x9d, Jpn. J. Appl. Phys., 35 (1996) pp. L217-L220.
It is known that a multi-quantum-well-structure using a thin film active layer can reduce a threshold value much, as compared with the case of a bulk active layer. In an InGaAlN based material, however, a threshold current density is still high and an operating voltage are also high and, therefore, there remain many problems to realize a continuous oscillation.
One of causes by which the operating voltage in the InGaAlN based material is high is that a contact resistance in the case of p-type is extremely high. In a stripe geometry of an electrode, which has been already reported, a voltage drop in a p-type electrode stripe is large and not only the operating voltage becomes high but also heat generation in the region cannot be neglected. In order to reduce a contact resistance, it is simply considered toexpand an electrode area, but in such a broad stripe geometry a magnitude of a threshold current becomes larger and a fundamental transverse-mode oscillation can not be available because a current injection region is large.
In application to an optical disc and the like, an output beam from a semiconductor laser is necessarily focused to a very small spot and therefore a fundamental transverse-mode oscillation is indispensable. However, in the InGaAlN based laser, a structure for a fundamental transverse mode stabilization has not been realized. In a conventional material system, for example an InGaAlP based system, only an SBR laser of a ridge stripe type has been reported in the following article:
3) M. Ishikawa et al.,: Extended Abstracts, 19th Conf. Solid State Devices and Materials, Tokyo (1987) pp. 115-118.
In an InGaAlN based laser, however, the structure used in the SBR laser cannot be applied without any change due to a difference in material system. As to a current confining structure in an InGaAlN based laser, a structure using GaN as a current confining layer is disclosed in the following application:
4) Jpn. Pat. Appln. KOKAI Publication No. 8-111558 (a semiconductor laser).
This structure works for current confinement but does not show a function of optical confinement and, thereby, there is difficulty producing an good quality output beam with small astigmatism and the like.
Generally, a composition, a thickness, a distance from an active layer and the like are necessarily set at respective designed values in order that a current confining layer formed in a cladding layer additionally works as a light confining layer. Especially in an InGaAlN based laser, even with the same composition, a completely different guiding mechanism is resulted in a laser according to a thickness and a position used because of a short wavelength. For this reason, an stable fundamental transverse mode oscillation is not provided only with incorporation of a current confining layer therein.
Also, if an Al containing layer is grown thick during a crystal growth process of an InGaAlN based materials, cracks are sometimes generated in the Al containing layer such as GaAlN layer, since there is a difference in lattice constant between an underlying GaN and the Al containing layer. For this reason, the transverse-mode confining in the direction of a layer (a vertical direction) does not work well so that a threshold value becomes extremately large or no guided mode is present.
On the other hand, a semiconductor laser used for an optical disc system, various specifications are specially required. Especially in write-once and rewritable types, a low power semiconductor laser for read and a high power semiconductor laser for erase/record are required and specifications for both are different from each other. Generally, a ultra thin film active layer is used for the high power semiconductor laser, but this structure is not necessarily suitable for a read laser. The reason why is that a low noise characteristic is required for the read laser and therefore, for example, a self-pulsation type structure is used, but the self-pulsation is hard to be obtained in the ultra thin active layer structure.
Under such circumstances, a high frequency superposition method or a combination of two kinds of laser has been adopted, but both are complicated in constitution. Besides, there has been reported a method, wherein two kinds of laser are formed using an active layer with variation of thickness according to positions, but in this method there arises a problem that controlling of thickness of the active layer is extremely difficult.
As described above, there have been proposed a variety of structures of and various methods of producing this kind of semiconductor laser, but no satisfactory characteristics have not been obtained in any cases, since crystal growth of a GaN based compound semiconductor layer is difficult. That is, the GaN based compound semiconductor layer cannot be grown as a good quality crystal and, therefore, carrier injection to an active layer cannot be effectively performed due to a poor crystal quality. In a structure wherein a stripe opening is formed in a current confining layer, a regrown layer after etching for formation of the stripe opening is degraded in crystal quality, which is an adverse factor for inviting a voltage drop in an electrode contact or the like.
To sum up, in order to realize a blue semiconductor laser with high reliability which works with a low threshold and a low voltage, and which is practically used for application of an optical disc and the like, effective current injection to an active layer and suppression of a voltage drop in a electrode contact or the like are important. However, in the current state of the art, a satisfactory structure with regard to the above points have not been obtained.
Moreover, semiconductor lasers of different wavelengths will be used more as an optical disc is progressed toward a higher density. In this trend, lasers of both wavelengths are sometimes in parallel needs, since interchangeability or compatibility is required between new and old optical systems. This situation will arise especially in the case where lasers have wavelengths of a large difference, such as in the case of a combination of red and blue in wavelength. The reason why is that a depth of a pit of an optical disc is optimized according to a wavelength of light source and, therefore, if a wavelength for read is greatly different, an SN ratio of a signal of reflection from a pit is reduced.
In such a way, in a conventional InGaAlN based semiconductor laser, a transverse-mode-stabilized structure is difficult to be produced and a laser which continuously oscillates in a fundamental transverse mode has difficulty being put to practical use.
For example, in an InGaAlN based semiconductor laser, it is difficult to produce a high-quality crystal layer because crystal growth thereof is difficult. A regrown layer after the etching for formation of a stripe opening has a further degraded crystallinity. For this reason, an efficiency in carrier injection to an active layer is reduced and, in addition, a voltage drop is caused by an electrode contact or the like. Accordingly, a device with high reliability which operates at a low threshold and a low voltage, and which is to be practically used for an optical disc and the like has difficulty being produced.
Moreover, degradation in crystallinity of a regrown layer on a GaN based compound semiconductor layer after the etching is held true to various semiconductor devices using a GaN based compound semiconductor.
Laser performances required for both cases of read and erase/record in an optical disc system are hard to be simultaneously realized.
There is difficulty in realization of a semiconductor laser which meet a need for compatibility between optical systems having different wavelengths of light sources and different recording densities.
It is an object of the present invention to provide an InGaAlBN based semiconductor laser, which can continuously oscillate in a fundamental transverse mode, and with which a good quality output beam with small astigmatism, and suitable for a light source in an optical disc system and the like, and a method of fabricating the same.
It is another object of the present invention to provide a semiconductor laser, which does not require a difficult process such as thickness control of an active layer, and which realizes a laser performance required in both operations of read and erase/record.
It is a further object of the present invention to provide a semiconductor laser which is required for maintaining compatibility between two optical systems designed to use respective different wavelengths of light sources, and which can be used in both optical systems.
It is a still another object of the present invention to provide a method of fabricating a semiconductor laser with high reliability, in which carrier injection to an active layer can be effectively conducted, in which a voltage drop in an electrode contact can be reduced, and which operates at a low threshold value and a low voltage for practical application in an optical disc and the like.
It is a still further object of the present invention to provide a method of fabricating a semiconductor laser in which regrowth after etching of a GaN based compound semiconductor layer can be performed in a satisfactory manner, and which contributes to improvements on characteristics of various kinds of semiconductor devices.
In order to achieve the above mentioned objects, in a semiconductor laser of the present invention, a light confining layer having a larger refractive index than that of a cladding layer is incorporated and a transverse mode is controlled with a loss or an anti-guiding effect in order to make possible a continuous oscillation in a stable fundamental transverse mode at a low operating voltage.
That is, the present invention is a semiconductor laser which is made of a III-V compound semiconductor including nitrogen, comprising: a first-conductivity-type cladding layer; a second-conductivity-type cladding layer having a ridge in the shape of a stripe; an active layer; a double heterostructure, in which the active layer lies between the first- and second-conductivity-type cladding layers; and a light confining layer formed adjoining the second-conductivity-type cladding layer side of the double heterostructure and at least in a region other than the ridge portion, wherein the light confining layer is made of a III-V compound semiconductor including nitrogen and a refractive index of the light confining layer is larger than that of the second-conductivity-type cladding layer.
The present invention is directed a semiconductor laser comprising: a first-conductivity-type cladding layer made of InxGayAlzB1xe2x88x92xxe2x88x92yxe2x88x92zN (0xe2x89xa6x, y, z, x+y+zxe2x89xa61); a second-conductivity-type cladding layer made of InuGavAlwB1xe2x88x92uxe2x88x92vxe2x88x92wN (0xe2x89xa6u, v, w, u+v+wxe2x89xa61) having a ridge in the shape of a stripe; an active layer; a double heterostructure, in which an active layer lies between the cladding layers; and a light confining layer formed on a surface of the second-conductivity-type cladding layer side of the double heterostructure and in a region thereof, which excludes at least a ridge portion, wherein the light confining layer is made of InpGaqAlrB1xe2x88x92pxe2x88x92qxe2x88x92rN (0xe2x89xa6pxe2x89xa61, 0xe2x89xa6q less than 1, 0xe2x89xa6rxe2x89xa61, 0 less than p+rxe2x89xa61, 0 less than p+q+rxe2x89xa61) and a refractive index thereof is larger than that of the second-conductivity-type cladding layer.
Here, preferred embodiments of the present invention will be described below:
(1) The active layer portion has a single-quantum-well-structure or multi- quantum-well-structure at least composed of a well layer made of InaGabAlcB1xe2x88x92axe2x88x92bxe2x88x92cN (0xe2x89xa6a,b,c,a+b+cxe2x89xa61) and a barrier layer made of IneGafAlgB1xe2x88x92exe2x88x92fxe2x88x92gN (0xe2x89xa6e, f, g, e+f+gxe2x89xa61).
(2) A thickness H1 of the first-conductivity-type cladding layer and a thickness H2 of the second-conductivity-type cladding layer are set, with respect to a total thickness d of a core region and an oscillating wavelength xcex of the laser, in ranges which satisfy the following relations;
0.18(zd/xcex)xe2x88x92xc2xdxe2x89xa6H1/xcexxe2x89xa60.27(zd/xcex)xe2x88x92xc2xd
0.18(wd/xcex)xe2x88x92xc2xdxe2x89xa6H2/xcexxe2x89xa60.27(wd/xcex)xe2x88x92xc2xd
(3) A total thickness dact of active layers is less than 0.05 xcexcm.
(4) A total thickness dact of active layers is equal to 0.045 xcexcm or less.
(5) An Al compositional ratio xAl of each of the cladding layers, an average compositional ratio YIn of the core region, a sum of the compositional ratios xcex94x (=xAl+YIn), a total thickness Hcore of a core region and a thickness Hclad of each of the cladding layers satisfy the following relation in reference to an oscillation wavelength xcex:
xcex94xxc2x7(Hcore/xcex)xc2x7(Hclad/xcex)xe2x89xa70.08
(6) Further, the above parameters satisfy the following relation;
xcex94xxc2x7(Hcore/xcex)xc2x7(Hclad/xcex)xe2x89xa70.1
(7) The above parameters satisfy the following relation;
xcex94xxc2x7(Hcore/xcex)xc2x7(Hclad/xcex)xe2x89xa60.2
(8) Further, the above parameters satisfy the following relation;
xcex94xxc2x7(Hcore/xcex)xc2x7(Hclad/xcex)xe2x89xa60.15
(9) An Al compositional ratio xAl and a thickness Hclad of each of cladding layers satisfy the following relation;
xAlxc2x7Hcladxe2x89xa60.1 xcexcm
(10) An Al compositional ratio xAl and a thickness Hclad of each of cladding layers satisfy the following relation;
xAlxc2x7Hcladxe2x89xa60.06 xcexcm
(11) The core region includes a plurality of waveguide layers made of InuGavAlwB1xe2x88x92uxe2x88x92vxe2x88x92wN (0 less than uxe2x89xa61, 0xe2x89xa6v less than 1, 0xe2x89xa6w less than 1) formed in such a manner that the active region lies between the waveguide layers, wherein a total thickness Hcore of the core region and an In average compositional ratio YIn of the core region satisfy, in reference to an oscillation wave-length xcex, the following relation;
(YIn)xc2xdxc2x7(Hcore/xcex)xe2x89xa70.15
(12) Further, the above parameters satisfy the following relation;
(YIn)xc2xdxc2x7(Hcore/xcex)xe2x89xa70.2
(13) A light confining layer has the same conductivity type as that of the second-conductivity-type cladding layer.
(14) A bandgap energy of the light confining layer is smaller than that of a bandgap of the active layer.
(15) A material of a contact layer on the second-conductivity-type cladding layer and the light confining layer are same and the cap layer having a bandgap between those of the second-conductivity-type cladding layer in a stripe region and the contact layer physically lies between both layers.
(16) Waveguide layers each having a refractive index smaller than that of an average refractive index of quantum-wells and larger than that of each of the cladding layers are disposed between the quantum-wells and the cladding layers and at least one carrier overflow blocking layer made of InsGatAlhB1xe2x88x92sxe2x88x92txe2x88x92hN (0xe2x89xa6s, t, h, s+t+hxe2x89xa61) and having a bandgap energy larger than that of each of the waveguide layers, is disposed in a waveguide layer on at least one side or between a waveguide layer and a quantum-well.
(17) An Al compositional ratio h of the at least one carrier overflow blocking layer is defined in the following relation;
0 less than h less than 0.2
(18) Each of the first-conductivity-type and second-conductivity-type cladding layers are made of GaAlN and the light confining layer is made of InGaN or GaAlN, an Al compositional ratio of which is smaller than that of each of the cladding layers.
For example, the light confining layer can be formed a structure as shown in one of the following conditions (i) to (iii).
(i) The light confining layer is made of Inp Ga9AlrB1xe2x88x92pxe2x88x92qxe2x88x92r N (0.2xe2x89xa6pxe2x89xa60.3, 0xe2x89xa6qxe2x89xa60.8, 0xe2x89xa6rxe2x89xa60.8, 0.2xe2x89xa6p+q+rxe2x89xa61).
(ii) The light confining layer is made of Inp Gag Alr B1xe2x88x92pxe2x88x92qxe2x88x92r N (0xe2x89xa6pxe2x89xa60.95, 0xe2x89xa6qxe2x89xa60.95, 0.05xe2x89xa6rxe2x89xa60.3, 0.05xe2x89xa6p+q+rxe2x89xa61).
(iii) The light confining layer is made of Ip Gaq Alr B1xe2x88x92pxe2x88x92qxe2x88x92r N (0xe2x89xa6pxe2x89xa60.95, 0xe2x89xa6qxe2x89xa60.95, 0.05xe2x89xa6rxe2x89xa60.1, 0.05xe2x89xa6p+q+rxe2x89xa61).
(19) A sapphire or SiC substrate is used as a substrate.
(20) A ridge portion of the second-conductivity-type cladding layer is formed concave downwardly or toward the substrate or in a reverse direction.
(21) An absorption coefficient of the contact layer is 100 cmxe2x88x921 or more.
(22) An absorption coefficient of the contact layer is 500 cmxe2x88x921 or more.
A semiconductor laser of the present invention, which emits two kinds of laser light, has a feature that it comprises: a first region, the first region having a double heterostructure portion including active layers of two kinds formed in a layered structure on a substrate; and a second region, the second region having a double heterostructure portion including one of the active layers, which is arranged on the side of the substrate, formed in a layered structure on the substrate, wherein the one of the active layers closer to the substrate has a bandgap larger than that of the other of the active layers farther away from the substrate.
In the above semiconductor laser, it is preferred that each double heterostructure portion comprises: a first-conductivity-type cladding layer made of InxGayAlzB1xe2x88x92xxe2x88x92yxe2x88x92zN (0xe2x89xa6x, y, z, x+y+zxe2x89xa61), and a second-conductivity-type cladding layer made of InuGavAlwB1xe2x88x92uxe2x88x92vxe2x88x92wN (0xe2x89xa6u, v, w, u+v+wxe2x89xa61) having a ridge in the shape of a stripe; an active layer sandwiched between the first- and second-conductivity-type cladding layers, wherein, in an region other than the ridge in the second-conductivity-type cladding layer, a light confining layer made of InpGaqAlrB1xe2x88x92pxe2x88x92qxe2x88x92rN (0xe2x89xa6pxe2x89xa61, 0xe2x89xa6q less than 1, 0xe2x89xa6rxe2x89xa61, 0 less than p+rxe2x89xa61, 0 less than p+q+rxe2x89xa61) with a refractive index larger than that of the second-conductivity-type cladding layer is formed.
A semiconductor laser of the present invention has another feature that the semiconductor laser is a semiconductor laser made of a GaN based compound semiconductor (InxGayAlzN: x+y+z=1, 0xe2x89xa6x, y, zxe2x89xa61) having a double heterostructure in which an active layer is sandwiched between cladding layers, wherein a ridge in the shape of a stripe made of a first-conductivity-type cladding layer and a first, first-conductivity-type contact layer is formed on at least one side of the cladding layer, a current blocking layer made of a second-conductivity-type GaN based compound semiconductor layer is formed in a region other than the ridge and adjoining the ridge, the current blocking layer and the ridge are buried in a second, first-conductivity-type contact layer to provide an electrode contact broader than a ridge width.
A method of fabricating a semiconductor laser of the present invention comprises at least: growing the first contact layer mentioned above in the state of crystal, forming SiO2, applying resist, forming a pattern having a stripe, forming a ridge by selective dry etching, forming a light confining layer (a current blocking layer) by selective growth using the SiO2 as a mask; removing the SiO2 mask on the ridges; and growing the second contact layer on the light confining layer (the current blocking layer) and on the ridges.
The method of the present invention has a feature that, in the dry etching step for the GaN based compound semiconductor(InxGayAlzN: x+y+z=1, 0xe2x89xa6x, y, zxe2x89xa61), a mixture of a first gas including at least chlorine as ingredient and a second gas including at least fluorine or oxygen as ingredient is used.
In this method, as to the first gas, Cl2, BCl3 or SiCl4 can be used and as to the second gas, CF4, C2F4, SF6, O2, CO or CO2 can be used. Dry etching with a mixture of these gases may be used in fabrication of semiconductor devices other than a laser.
According to an InGaAlBN based semiconductor laser of the present invention, a ridge is formed in a cladding layer of one side of a double heterostructure and in a region other than the ridge a light confining layer made of InpGaqAlrB1xe2x88x92pxe2x88x92qxe2x88x92rN (0xe2x89xa6pxe2x89xa61, 0xe2x89xa6q less than 1, 0xe2x89xa6rxe2x89xa61, 0 less than p+rxe2x89xa61, 0 less than p+q+rxe2x89xa61) with a refractive index larger than that of the cladding layer is formed. With the light confining layer, current confining is effected and at the same time optical confinement by a refractive index distribution is formed to control a transverse mode and, thereby, a threshold current density is reduced, which makes a continuous oscillation in a fundamental transverse mode possible.
Here, the light confining layer as mentioned above has been considered to be difficult to be selectively grown, since a lattice constant is much different from that of the double heterostructure. Therefore, in a conventional InGaAlN based semiconductor laser, no technical concept has been present that a light confining layer is formed in a region other than a ridge in a cladding layer. However, the inventors of the present invention has optimized various growth conditions in a metalorganic chemical vapor deposition method (MOCVD) or a molecular beam epitaxy method (MBE) through experiments in their serious research to make it clear for selective growth of the light confining layer to be practically possible.
In addition, it has been also made clear that in a structure having an InGaAlBN based light confining layer with an In compositional ratio of 0 or more, a carrier density in a cladding layer on the lower side of the light confining layer is increased. The reason why has been found to be that inactivation of a Mg acceptor by hydrogen and the like is suppressed and thereby carrier overflow is much reduced, as compared with a structure without a light confining layer. Moreover, by disposing a light confining layer of the present invention, more reduction in a threshold current density than conventionally possible and a continuous oscillation in a fundamental transverse mode has become a reality in a practical sense.
According to a semiconductor laser of the present invention, a low power laser of a thick film active layer and a high power laser of a thin film active layer are formed on the same substrate and therefore a laser performance which is required for both of read and erase/record in an optical disc system is realized in the semiconductor laser without any complicated process such as a thickness control of an active layer or the like.
Moreover, according to a semiconductor laser of the present invention, lasers of different wavelengths are formed on the same substrate and therefore a problem of non-compatibility originating from a difference in wavelength can be solved.
There will be described the relation between the thickness tB of the barrier layer and the thickness tw of the well layer in the core region. If the thickness tB is larger than the thickness tw, the light confining effect is increased and the guided mode loss is decreased. If the thickness tB is not larger than the thickness tw, the carriers are uniformly injected into the well layer, as a result, the threshold current can be reduced.
According to a method of the present invention, even with a thin contact layer, an opening of a current confining layer can be buried in it in a planar structure. That is, since the contact layer can be made thin, a device resistance in the contact layer is kept low and, since an electrode contact is fabricated planar, a crystallinity is good. A voltage drop at the electrode contact is suppressed and current injection is uniformly effected, so that improvements toward a low threshold and on reliability are achieved.
The above and other objects, and features and advantages of the present invention will be more apparent from the following detailed description taken in connection with the accompanying drawings, wherein, in the drawings, reference marks similar to each other show parts equivalent to each other.
Additional object and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.